1. Field of the Invention
The present invention relates to a power supply (or power source) monitoring device, and in particular to a power supply monitoring device monitoring a power supply voltage supplied to a monitored circuit to protect the monitored circuit from an operational malfunction which occurs due to a reduction of the power supply voltage or the like.
2. Description of the Related Art
As the above-mentioned power supply monitoring device, the following prior art examples [1] and [2] have been known.
Prior Art Example [1]: FIGS. 14A and 14B
A power supply monitoring device 10 shown in FIG. 14A is composed of a fuse/electromagnetic wave filter 100 which is connected to a ground GND and is supplied with an input voltage Vin (e.g. “−48 V”) for the whole device, onboard power supplies 200_1-200_3 (hereinafter, occasionally represented by a reference numeral 200) respectively receiving an output voltage Vout from the fuse/electromagnetic wave filter 100 as the input voltage Vin and supplying voltages Vcc_1-Vcc_3 (hereinafter, occasionally represented by a reference character Vcc) of e.g. “+1.2 V”, “+1.8 V”, and “+3.3 V”, power supply voltage monitoring portions 300_1-300_3 respectively monitoring the power supply voltages Vcc_1-Vcc_3, a monitored circuit 400_1 operated with the power supply voltages Vcc_1 and Vcc_3, a monitored circuit 400_2 operated with the power supply voltages Vcc_2 and Vcc_3, a monitored circuit 400_3 operated with the power supply voltage Vcc_3, and a pull-up resistor 600 commonly connected to the power supply voltage monitoring portions 300_1-300_3.
Also, the onboard power supplies 200_1-200_3, the power supply voltage monitoring portions 300_1-300_3, and the monitored circuits 400_1-400_3 are commonly connected to a signal ground SG. Each reset input RST of the monitored circuits 400_1-400_3 is commonly connected to each output of the power supply voltage monitoring portions 300_1-300_3.
In operation, as shown in FIG. 14B, the power supply voltage monitoring portions 300 perform reset processing to all of the monitored circuits 400 since there is a possibility that an operational malfunction is occurring in the monitored circuits 400 when any one of the power supply voltages Vcc_1-Vcc_3 is reduced down to a voltage V4 lower than a voltage V1 guaranteeing the operation of the monitored circuits 400.
Thus, it is made possible to protect the monitored circuits 400 from the operational malfunction occurring due to a reduction of the power supply voltage Vcc.
Prior Art Example [2]: (Not Shown)
There is a power supply monitoring device, which will be described referring to FIGS. 14A and 14B. This power supply monitoring device stops an operation of the monitored circuits 400 shown in the above-mentioned prior art example [1] to perform abnormality processing (saving active data in memory or the like) when the power supply voltage Vcc is reduced down to a voltage set higher than the voltage V1 guaranteeing the operation of the monitored circuits 400, and performs the reset processing to the monitored circuits 400 when the power supply voltage Vcc is further reduced down to the voltage V1 (e.g. see patent document 1).
[Patent document 1] Japanese Patent Application Laid-open No. 7-93066
While an IC circuit or the like is generally used for the power supply voltage monitoring portion in the prior art example [1] shown in FIG. 14A, there are variations (detection errors) for a detection accuracy of the power supply voltage per circuit.
In consideration of such detection errors, in the above-mentioned prior art example [1], the power supply voltage monitoring portion sets a voltage detecting a malfunction of the monitored circuit so as to have a margin for errors, compared with a voltage guaranteeing the operation of the monitored circuit. Therefore, there has been a problem that an occurrence of an operational malfunction in the monitored circuit can not be instantaneously detected by the reduction of the power supply voltage, and services provided by the device are kept stopped for a long time.
Also, in the above-mentioned prior art example [2], the operation of the monitored circuit is stopped at the time when the power supply voltage is reduced down to the voltage set higher than the voltage guaranteeing the operation of the monitored circuit. Therefore, when the power supply voltage is instantaneously interrupted, suddenly reduced, or the like (namely, when the monitored circuit recovers after the power supply voltage is reduced down to the voltage for stopping the operation of the monitored circuit, but not to the voltage for performing the reset processing to the monitored circuit), the operation of the monitored circuit is kept stopped regardless of the presence or absence of the operational malfunction. Consequently, there has been a problem that the services provided by the device are stopped even when stopping the services is not necessary.